Within the semiconductor industry, lithographic printers use reticles (also called masks) having device patterns to pattern photoresist layers. The reticle comprises a substrate, typically quartz, with an opaque layer, e.g., chrome, thereon. State of the art semiconductor devices require very small dimensional patterns. These small patterns can be formed within a photoresist layer as long as the pattern is within the resolution of the printer, which is herein defined as the smallest dimension that can be resolved within the photoresist layer while maintaining an acceptable process window. The practical resolution is approximately: ##EQU1## where k.sub.1 is a "constant" for a given lithographic process (process constant), .lambda. is the wavelength of the radiation, and NA is the numerical aperture of the lens. One skilled in the art appreciates that k.sub.1 is not a true constant, but can actually vary. A conventional reticle has a k.sub.1 of approximately 0.8.
When exposing a feature having a dimension near the practical resolution with a conventional reticle, diffraction effects become significant. Because of this, portions of the photoresist underneath an opaque section near the interface of the opaque section and an opening in the reticle receive some of the exposing radiation. In order to minimize this problem, phase-shifted reticles have been used in the prior art.
In the following discussion a phase-shifted reticle used to pattern a positive photoresist will be used for illustration. Portions of the chrome layer will be referred to as "chrome elements." "Openings" within the chrome layer are regions from which the chrome layer has been removed, which transmit a substantial fraction of the exposing radiation, and which correspond to the pattern to be formed on the photoresist layer. "Phase-shifting elements" are regions between the chrome elements and openings from which the chrome has been removed and which transmit a substantial fraction of the exposing radiation, but shift the phase of the radiation approximately 180.degree. relative to the opening. Generally, the phase difference of 180.degree. between the opening and a phase-shifting element is created by a difference in the thickness of the quartz substrate between the two regions, or by the addition of a thickness of a material such as spin-on-glass (SOG), for example. A typical prior art phase-shifted reticle is shown in FIG. 1 and includes quartz plate 10 having chrome element 11, phase-shifting rim 12, and opening 13. A phase-shifting rim is a phase-shifting element which surrounds an opening such as opening 13. A "pattern" on a reticle as used herein is that portion of the reticle which forms an image in the developed photoresist pattern. In prior art non phase-shifted reticles, the pattern on a reticle consists of the opening in the chrome layer. In a phase-shifted reticle, the pattern consists of both the opening as well as any phase-shifting element associated with the opening. For example, the pattern shown in FIG. 1 comprises opening 13 and phase-shifting rim 12. The pattern formed by the portion of a reticle shown in FIG. 1 could be a contact or via opening. This pattern will be referred to as an "opening pattern" in the present specification.
Within this application, a phase-shifting element width is sometimes expressed as a fraction of IRF..lambda./NA where IRF is the image reduction factor of the lens, .lambda., is the wavelength of the radiation, and NA is the numerical aperture of the lens. Widths expressed in units of IRF..lambda./NA are used because the actual width of a phase-shifting element varies based on those three parameters. Although k.sub.1 is a function of the phase-shifting element width, an equation to determine k.sub.1 given a phase-shifting element width is not known. Generally, k.sub.1 decreases as the phase-shifting element width increases, but it is understood that k.sub.1 may not be a linear function of the phase-shifting element width. Therefore, an increased phase-shifting element width results in a decreased value of k.sub.1, allowing for improved resolution. However, at least one process complication occurs. With an increased phase-shifting element width, the radiation intensity at the photoresist surface under the phase-shifting element increases. This can cause undesired exposure of the photoresist under the phase-shifting element. When the prior art phase-shifted reticle has a phase-shifting element with a width greater than about 0.4 IRF..lambda./NA, the phase-shifting element is too wide, and the photoresist layer under the center of the phase-shifting element is substantially exposed when the reticle is exposed to radiation. The phase-shifting element width is usually no less than about 0.1 IRF..lambda./NA because k.sub.1 for a phase-shifting element width less than 0.1 IRF..lambda./NA is close to the same value as k.sub.1 for the conventional reticle.
In FIG. 1, the phase-shifting rim 12 has a thickness such that radiation transmitted through the phase-shifting rim 12 is shifted about 180.degree. out of phase relative to the radiation transmitted through the opening 13. The transmittance of radiation transmitted through the opening 13 is about the same as the transmittance of radiation transmitted through the phase-shifting rim 12.
The phase-shifted reticle is used to pattern a positive photoresist layer as illustrated in FIGS. 2A, 2B, and 2C. FIG. 2A includes a cross-sectional view of the reticle of FIG. 1 and has quartz plate 10, chrome element 11, phase-shifting rim 12, and reticle opening 13. When radiation is incident on the reticle, the radiation is transmitted through the reticle opening 13 and the phase-shifting rim 12, but the chrome clement 11 prevents virtually all transmission of radiation. FIG. 2B illustrates how the radiation transmitted through the reticle divided by the radiation incident on the reticle, I/I.sub.0, may vary across the photoresist layer surface when using the reticle of FIG. 1. As seen in FIG. 2B, I/I.sub.0 under the chrome element 11 is substantially zero, and I/I.sub.0 under opening 13 away from the phase-shifting rim 12 is close to unity.
Interference areas A21 and A22 each include a portion of the phase-shifting rim 12 as shown in FIG. 2A. Within each interference area, the radiation transmitted through the phase-shifting rim 12 is shifted about 180.degree. out of phase relative to the radiation transmitted through the reticle opening 13. Radiation from the reticle opening 13 that enters the interference areas is interfered with by the radiation that is transmitted through the phase-shifting rim 12 within the interference areas.
The actual patterned photoresist layer typically has at least one problem. As seen in FIG. 2B, some radiation reaches the photoresist layer under the phase-shifting rim 12. I/I.sub.0 beneath the center of the phase-shifting element increases as the width of the phase-shifting element increases. After developing, the photoresist layer has resist elements 18 each with a recession 23 near a photoresist layer opening 22 as shown in FIG. 2C. To solve the problem of the recession caused by the prior art reticle of FIG. 1, a phase-shifted reticle having phase-shifting elements with a reduced transmittance has been discovered. See co-pending U.S. patent application Ser. No. 869,026; filed Apr. 15, 1992; entitled "Lithography Using A Phase-Shifted Reticle With Reduced Transmittance," which application is assigned to the assignee of the present invention.
As dimensions are scaled for increased device density, the dimensions and spacing of contact patterns such as those shown in FIG. 1 can be expected to decrease. A problem will be encountered with the prior art rim phase-shifted reticle of FIG. 1 for forming contact openings of small dimensions on scaled devices. Often, it is desired to put the contacts in an array. In an array, two opening patterns may be in close proximity, or may be adjacent to one another along a side. In this case, the two close or adjacent phase-shifting rims are roughly equivalent to one very wide rim. As noted above, as the phase-shifting rim width is increased, I/I.sub.0 underneath the phase-shifting element increases. FIG. 3 shows a reticle with two prior art phase-shifting opening patterns placed side by side. FIG. 3B shows a plot of I/I.sub.0, plotted along the line 15-15a of FIG. 3A, showing the intensity of the exposing radiation at the photoresist surface. At the outer edges near the points 15 and 15a, the destructive interface between phase-shifting rim 12 and opening 13 has significantly reduced the transmitted light intensity. However, the intensity of radiation between the openings 13 is significantly higher. The increased intensity will cause a deep recession in the exposed photoresist, and may in fact cause a portion of the photoresist to be removed between the contact or via openings. Ibis problem occurs not only when two patterns are adjacent along a side as shown in FIG. 3A, but also when the patterns are in close proximity to one another. Even if a strip of chrome is between the two adjacent phase-shifted rim opening patterns, the increased intensity between the two patterns will still be too great if they are positioned at approximately 0.55 .lambda./NA or less.
What is needed is a phase-shifted reticle allowing for patterns having a phase-shifting rim or phase-shifting element along one or more sides to be placed closely together without causing an unacceptable increase in the exposure intensity between the patterns.